The present invention generally relates to operational amplifiers and, more particularly, to a switch capacitor common mode feedback (CMFB) circuit portion for a fully differential telescopic operational amplifier for reducing systematic current mirror error, the operational amplifier being particularly suited for use as a sample and hold stage or a residue amplifier stage in a high resolution pipeline analog to digital converter.
FIG. 1 illustrates a traditional switched capacitor common mode feedback (CMFB) circuit portion 10 for a differential telescopic operational amplifier. The operational amplifier has an input differential pair made up of MOSFETs M3 and M4 having gates respectively connected to input nodes In+ and Inxe2x88x92, where In+ and Inxe2x88x92 are the non-inverting and inverting inputs of the operational amplifier. MOSFET M1 is connected to mirror current to M2 such that bias current I3 is approximately equal to the sum of bias currents I1 and I2. The current mirroring uses a switched current technique via switch 12a. Switch 12a is implemented with CMOS transmission gates enabled by a non-overlapping clock signal. In the context of a sample and hold amplifier, the clock signal defines a sample period and a hold period. More specifically, during the sample period, switches 12a, 12b and 12c close and capacitors CCMFB+ and CCMFBxe2x88x92 are charged to a voltage approximately equal to the output common mode voltage during the sample period (VOCM, sample) minus the gate to source voltage of the pair M1 and M2 (VGS,M1/M2). During the hold period, switches 12a, 12b and 12c open and M2 along with the capacitors CCMFB+ and CCMFBxe2x88x92 form a common mode feedback loop for the telescopic amplifier.
The traditional switched capacitor CMFB portion 10, however, suffers from the introduction of error in current mirroring (i.e., Ierror) such that the bias current I3 plus lerror no longer is approximately equal to I1 plus I2. For no systematic current mirroring error due to channel length modulation, the drain to source voltage of M1 (i.e., VDS,M1) should, theoretically, be equal to the drain to source voltage of M2(i.e, VDS,M2). However, in the traditional switched capacitor CMFB portion 10, the drain to source voltage of M1 is equal to the gate to source voltage of M1 (i.e., VDS,M1=VGS,M1) and the drain to source voltage of M2 is equal to the input common mode voltage (VICM) of the differential input signal minus the gate to source voltage of differential pair M3/M4 (i.e., VDS,M2=VICMxe2x88x92VGS,M3/M4). If VDS,M1 does not equal VDS,M2, a systematic error in current mirroring due to channel length modulation is to be expected, which can cause the output common mode voltage during the hold period (VOCM hold) to deviate significantly from to move significantly from VOCM hold. It is noted that VOCM, hold is equal to (VOUT++VOUTxe2x88x92)/2. To reduce channel length modulation, it is known to cascode both M1 and M2. However, to accommodate high output voltage swing (e.g., a swing of about 1.5 V where VDD equals 3.0 V), the M1/M2 current mirror is not cascoded. It is also known to increase the lengths of M1 and M2 to decrease channel modulation effect. However, increases in length are limited due to a corresponding increase in the gate capacitance of M2 which, if too great relative to the capacitances of CCMFB+ and CCMFBxe2x88x92, can reduce the common mode feedback loop gain.
FIG. 2 illustrates another switched capacitor CMFB circuit portion 20 for a telescopic operation amplifier using a more elaborate current mirroring technique to accommodate high voltage swings. For a detailed discussion of the input stage 20, attention is directed to Gulati, Kush and Lee, Hae-Seung, A High-Swing CMOS Telescopic Operational Amplifier, IEEE Journal of Solid-State Circuits, Vol. 33, No. 12, pp.2010-2019, December 1998.
Briefly, the current mirroring technique of the switched capacitor CMFB portion 20 uses a technique referred to as replica loop feedback. More specifically, the replica loop feedback replicates the differential pair M3/M4 for M1 as differential pair M5/M6. In addition, a negative feedback circuit using an operational amplifier 22 and a compensating capacitor (not shown) is provided to force the drain voltage of M5/M6 to be equal to the bias voltage. The compensating capacitor for the operational amplifier 22 has a capacitance approximately equal to the gate capacitance of M1. Although the current mirroring for the switched capacitor CMFB portion 20 does provide good minimization of current mirror error, it suffers from the disadvantage that extra circuitry to implement the operational amplifier 22 is required.
The present invention provides a fully differential telescopic operational amplifier having a switch capacitor common mode feedback (CMFB) circuit portion. The switched capacitor CMFB circuit portion has a current mirror for mirroring current from a first transistor to a second transistor. The first and second transistors have gates which are coupled via a transmission gate switch. Drains of each of the first and second transistors are respectively coupled to a first and a second differential pair of transistors, each differential pair connected to receive a common mode voltage and drains of the first differential pair connected to the gate of the first transistor. The first and second differential pair are scaled with respect to each other such that the gate to source voltage of the first transistor is substantially equal to the gate to source voltage of the second transistor.